In recent years, the miniaturization of switching power supplies has been an important development tendency, but magnetic elements have occupied a fairly large proportion of switching power supplies in terms of volume, weight, loss and cost, etc. Increasing the frequency of switching power supplies is an effective means of reducing the volume of magnetic elements and increasing the power density of switching power supplies, and it is also a hotspot of magnetic design at present. As frequency rises, both a magnetic core loss and a winding loss increase sharply. Thus, decreasing the magnetic core loss and the winding loss is more important for the analysis and design of magnetic devices.
Center-tapped transformer is widely used in power converter with two secondary windings to perform dual half-wave rectification. Different from a non-center tapped transformer, the currents flowing through the two secondary windings of the center-tapped transformer are not synchronous, i.e., the two secondary windings work at a time-sharing mode. An eddy current loss still can be induced in one of the two secondary windings through which current doesn't flow. Due to the particularity of the currents flowing through the two secondary windings in a center-tapped transformer, the analysis of its loss is different from an ordinary non-center tapped transformer.
For a center-tapped transformer, it is a common practice in the prior art that the copper sheet for each layer of the primary and secondary windings has the same thickness. Although it is convenient to design and manufacture such a center-tapped transformer, it is not the best choice in terms of loss and volume.
Refer to FIG. 1A and FIG. 1B. FIG. 1A is an equivalent circuit diagram of an existing center-tapped transformer applied to a LLC circuit, and FIG. 1B is an equivalent circuit diagram of an existing center-tapped transformer applied to a PWM circuit. The dashed boxes in the figures indicate the transformers. P indicates the primary winding of the transformer. S1 and S2 respectively indicate the two secondary windings of the transformer, and the two secondary windings do not work simultaneously. As shown in both FIG. 1A and FIG. 1B, K1 and K2 as well as K3 and K4 (only shown in FIG. 1B) are switching transistors of the primary side of the transformer, D1 and D2 are the switching transistors of secondary side, Co and Cs each indicates capacitance, Ls and Lm each indicates inductor, and Ro indicates output resistor. FIGS. 2A to 2C are the current waveforms of the various windings of the existing center-tapped transformer applied to the LLC circuit, wherein the current waveform of the primary winding in single cycle is a sine-like wave, and the current waveform of each of the two secondary windings in single cycle is a sine-like wave in a half cycle and is 0 in the other half FIGS. 3A to 3C are the current waveforms of the various windings of an existing center-tapped transformer applied to a PWM circuit, wherein the current waveform of the primary winding in single cycle is a positive-negative pulse-like wave, and the current waveform of each of the two secondary windings in single cycle is an pulse-like wave in a half cycle and is 0 in the other half. Thus it can be seen that in both the LLC circuit and the PWM circuit, the current waveforms of the primary winding P in single cycle have a positive part and a negative part. Therefore the fundamental harmonic component of the current in the primary winding P is high, and the DC component and the higher harmonic component are low so that they can be ignored. While currents in the two secondary windings S1 and S2 do not work simultaneously. In single cycle, there is current in only a half cycle while there is no current in the other half Therefore the fundamental harmonic component and the DC component of the currents in the two secondary windings S1 and S2 are high, while the higher harmonic component is low so that it can be ignored, too. Due to the characteristics of harmonic components of the currents flowing through the primary and secondary windings of the center-tapped transformer, how to reasonably set the thicknesses of copper sheets for the primary and secondary windings to distinguish it from a center-tapped transformer in the prior art, in order to make the fullest use of the limited window area, is a problem to be solved by those skilled in the art.